This invention relates to waveguide filters, and more particularly to tuneable E-plane waveguide filters.
E-plane waveguide filters consist of a waveguide, formed by two halves of a rectangular parallelepiped housing, and an insert. The insert is a relatively thin sheet of electrically conductive material, typically copper, of uniform thickness and etched or stamped with patterns. The insert is placed between the two halves of the housing so that when the waveguide is assembled the insert lies along the longitudinal axis of the waveguide and is oriented in a plane parallel to the short dimension of the cross-section of the waveguide. The patterns in the insert consist of spacings, or cavities, separated by remaining portions of the insert called fins, all of which run the full interior height of the waveguide. The cavities have resonant frequencies defined by their geometry and the fins have inverting properties defined by their geometry. The frequency response of the filter depends on the lengths of the cavities and fins, on the thickness of the insert, and on the dimensions of the waveguide housing.
Manufacturing tolerances on the waveguide housing and on the etching or stamping of the insert place limits on the precision of the filter dimensions, and existing E-plane waveguide filters are unable to provide the precise frequency response needed for narrow bandwidth filters. One solution is to improve the manufacturing process for creating the waveguide housing and the insert in order to improve the precision in the dimensions of the waveguide filter. However this is expensive for the precision needed for narrow bandwidths. Another solution is to fine-tune the filter after manufacture to achieve the desired frequency response from the filter. H-plane filters can be tuned after manufacture, but these are more expensive than E-plane filters due to the more complex assembly required. Furthermore, the tuning of H-plane filters is complex, requiring the adjustment of many tuning screws. There is a need for tuneable E-plane waveguide filters, as these would be less expensive than H-plane filters yet would allow narrower bandwidth filters.
The present invention provides a waveguide filter comprising an electrically conductive waveguide housing containing a longitudinally extending rectangular channel having spaced sides, the housing being constructed of at least two housing portions assembled together, and at least one electrically conductive relatively thin planar insert extending along and spaced from the sides of the waveguide channel. The upper and lower edges of the insert are sandwiched between two of the housing portions. The insert has at least one cavity located between the upper and lower edges of the insert and situated in the waveguide channel. A recess is provided in the insert, extending from one of the upper and lower edges of the insert into the cavity, and a tuning slider of electrically conductive material is received in the recess and extends into the cavity a distance determined by the desired frequency response of the waveguide filter. The presence of the slider alters the resonant frequency of the cavity, thereby changing the frequency response of the filter. The thickness of each slider and the approximate distance each is to extend into a cavity in the insert is determined analytically. Once inserted, the position of each slider is finely adjusted until the measured frequency response is as desired, and the sliders are then fixed in position.
The recess in a preferred embodiment passes through the entire thickness of the insert to form a gap. It is noted that because the tuning technique is effected by modifying the insert, it is not necessary to modify or alter the housing portions, thus permitting the use of a universal housing for a range of filter designs.
In an alternative embodiment, instead of providing a recess in the insert, a notch is provided in a wall of a housing portion and tuning can be achieved by receiving the slider in the notch. This solution is less preferable because it requires modification of the standard housing.
This construction of waveguide filter allows very precise frequency response curves to be obtained, overcoming the limits imposed by manufacturing tolerances, without the complexity and cost of an H-plane waveguide filter. Furthermore, a particular waveguide filter can later be tuned to a slightly different frequency response by adjusting the sliders.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.